The Institute of Electrical and Electronics Engineers (IEEE) 802.1D Spanning-Tree Protocol (STP) standard is a link management protocol and provides distributed routing over multiple Local Area Networks (LANs) that are connected by Media Access Control (MAC) bridges. The 802.1D standard is presented in detail in IEEE Standard for Local and Metropolitan Area Networks—Common Specification, Part 3: Media Access Control (MAC) Bridges (The Institute of Electrical and Electronics Engineers, Inc., New York, N.Y. 1998), which is hereby fully incorporated herein by reference.
The STP protocol provides path redundancy, while preventing undesirable loops in a network that are created by multiple active paths between bridges. Loops occur when there are alternate routes between hosts. To establish path redundancy, STP creates a tree that spans all of the bridges in an extended network, forcing redundant paths into a standby, or blocked, state. STP allows only one active path at a time between any two network devices (this prevents the loops), but establishes the redundant links as a backup path if the initial link should fail. If STP costs change, or if one network segment in the STP becomes unreachable, then STP reconfigures the spanning tree topology and reestablishes the link by activating the standby path. Without the spanning tree in place, it is possible that both connections may be simultaneously live, which could result in an endless loop of traffic on the Local Area Network(LAN).
For any network topology changes, the convergence time in the STP (IEEE 802.1D) standard is usually about 50 seconds (i.e., two times the forward delay plus a maximum age time).
However, there is a need for further enhancements and optimizations to the implementation of the STP (IEEE 802.1D) standard.